In Kate Greenaway's Language of Flowers, the entry for Acacia, Yellow reads: Secret love.
Why the color qualifier matters
The yellow-acacia entry changes the message attached to acacia in this dictionary. Its emphasis is concealed affection, whereas the general acacia entry concerns friendship. That distinction matters when reading an old floral list or designing a historically referenced arrangement.
For a present-day gift, discretion should not become an expectation that the other person will understand a hidden declaration. If a message is important, include it directly and let the historical association remain an optional layer.
FAQ
Is this a recommendation to send a secret romantic message?
No. The entry explains a dictionary convention; a recipient's comfort and your actual words matter more.
Source
Kate Greenaway: Language of Flowers, plant-to-meaning entries